Hot blast stoves, sometimes referred to as blast furnace stoves, are typically employed in iron manufacturing to preheat combustion air before it enters into a blast furnace. A hot blast stove typically has a cylindrical, silo-shaped wall structure constructed of refractory and insulating brick, and surrounded by a metal shell. Adjoining combustion and checker chambers are defined by a vertically extending internal dividing wall also constructed of refractory materials. The chambers communicate through a passage formed adjacent a dome at the top of the cylindrical structure. The dome protects the steel shell at the top of the blast stove from excessive high temperatures. The dome in a hot blast stove is typically supported either by an extended diameter steel support structure with steel supports or, in the case of an internal dome, by means of the cylindrical wall.
The checker chamber, also referred to as a regenerative chamber, includes tiers of refractory brick having aligned flow passages which extend from the top to the bottom of the chamber. The bricks absorb and store heat from hot exhaust gases which pass through the checker chamber during a heating cycle. The hot gases flow upwardly in the combustion chamber and then travel downwardly through the checker chamber and exit at the bottom of the checker chamber. Once the checker brick has attained a predetermined temperature, the heating cycle is terminated and the blast cycle begins. In the blast cycle, outside air is introduced at the bottom of the checker chamber and travels upwardly and absorbs the stored heat. This preheated air then travels down through the combustion chamber, exits the stove, and enters the blast furnace.
The internal operating temperature in the blast stove varies considerably and is well in excess of 2000° F. in certain portions of the chamber. In the internal dome structure described above, the wall on the combustion chamber side of the blast stove expands faster and thermally cycles more, causing significant expansion and contraction during normal operating cycles, as compared with the wall on the checker chamber side of the blast stove. This difference in expansion over the large height of the blast stoves, typically 200, 300 or more feet tall, contributes to the formation of cracks in the dome and often leads to premature dome failure. Once the hot face of the refractory dome starts to crack, insulation between the dome and the metal shell is compromised. This results in local hot spots on the steel shell. Typically, to cope with these hot spots, the blast stove must be isolated from the blast furnace to conduct repairs. Such repairs can be done by accessing the stove from the outside, requiring scaffolding on the outside of the stove over large heights, typically 200 to 300 feet or more. Commonly, strategic locations are identified on the shell and openings are drilled to weld grout nipples on the shell in the vicinity of a hot spot. The grout nipples are connected to a pump which injects a semi-plastic refractory insulating material into the area. This method is often used many times during the life span of a stove to keep the stove shell from over-heating in the vicinity of a cracked dome. In some cases, the heavy cracking is so excessive and damage on the inside of the dome is so large that locally the dome collapses and repairs on the inside are required. To facilitate these repairs, the blast stove needs to be isolated from the blast furnace and cooled to ambient temperatures to allow access to the inside. All of these described repairs significantly contribute to financial loss due to maintenance costs and the inability to operate the blast stove during the repair maintenance.
In conventional blast stoves, various measures have been taken in attempts to avoid thermal damage to the dome resulting from expansion differences in the outer dome supporting blast stove wall. Typically, the outer wall of the blast stove in the combustion chamber area is provided with both an additional insulation wall and a dense refractory wall inside the dome supporting wall. These additional walls provide additional insulation of the combustion chamber supporting wall to reduce the expansion of the dome supporting wall on the combustion chamber side and equalize its expansion to that of the cooler dome supporting wall on the checker chamber side. Not only does this design require additional engineering, material and construction, its effect in preventing dome cracks and deterioration of the dome structure over the life of the blast stove has been limited as variations in the thermal expansion of the supporting wall in the area of the combustion chamber still occur and often cause significant dome cracking.
Accordingly, there is a need for improved hot blast stove design which overcomes one or more disadvantages of the conventional designs.